A Grade 8 buoyancy worksheet with 23 questions (30 marks).
Students:
read spring scales in air and in water to find the buoyant force
use force diagrams to decide whether an object floats, sinks or rises
solve Archimedes’ principle problems
answer true/false questions
explain ships, salt water and submarines
analyse a spring-scale investigation
fix a common mistake
A worked answer key is included.
By Kris Reddy, Ontario teacher · Updated October 17, 2026 · 23 questions (30 marks) · PDF + answer key
This free buoyancy worksheet helps Grade 8 students understand why things float and sink. Students read drawn spring scales in air and underwater to find the buoyant force, use force diagrams to decide whether an object floats, sinks or rises, and apply Archimedes' principle to stones, blocks, toy boats and balls.
On the second page they sort true and false statements, explain cargo ships, swimming in the ocean and submarines, analyse a spring-scale investigation, and fix a common "bigger rock, bigger buoyant force, so it floats" mistake. Every answer was checked by computer. To print it, use the Download button above.
The buoyancy worksheet supports the Ontario Grade 8 science curriculum, Matter and Energy: Fluids, where students explain buoyancy and how it applies to everyday objects and technologies.
What Is Buoyant Force?
Any fluid, liquid or gas, pushes up on an object placed in it. This upward push is the buoyant force. You can measure it with a spring scale: weigh an object in air, then weigh it fully underwater. The scale reads less underwater, and the difference is the buoyant force.
10 N in air, 7 N in water, so the buoyant force is 10 − 7 = 3 N.
The underwater reading is called the apparent weight: weight in air − buoyant force. Part A of the buoyancy worksheet has students read three pairs of scales like these.
Archimedes' Principle
Buoyant force = weight of the fluid displaced
When an object goes into water, it pushes some water out of the way. Archimedes discovered that the upward push equals the weight of that displaced water. To keep the numbers simple, the worksheet uses 100 mL of water weighs 1 N (1 kg weighs about 10 N). So a stone that displaces 250 mL feels a buoyant force of 2.5 N. A floating object displaces exactly its own weight of water, which is why a 3 N toy boat displaces 300 mL. Read more about Archimedes' principle on Wikipedia.
Float, Sink or Rise?
Two forces act on an object in water: gravity (its weight) pulls down, and the buoyant force pushes up. Compare them:
Balanced (6 N = 6 N): floats · weight bigger (10 N vs 4 N): sinks · buoyant force bigger (7 N vs 3 N): rises
This is the force view of what the density worksheet teaches: an object less dense than water can displace its own weight before it's fully under, so it floats. Part B of the buoyancy worksheet asks students to find the net force for each diagram.
Ships, Submarines and Salt Water
Ships: a steel hull is shaped to displace a lot of water. A loaded ship sits lower because it must displace more water to balance its extra weight.
Salt water: it's denser than fresh water, so the same displaced volume weighs more and gives a bigger buoyant force. That's why floating is easier in the ocean.
Submarines: ballast tanks fill with water to make the sub heavier (dive) or fill with air to make it lighter (surface).
Hot-air balloons and life jackets use the same idea in air and water.
Buoyancy Worksheet Practice Questions
These questions match the buoyancy worksheet PDF, which includes the drawn spring scales, force diagrams and data table.
Part A – Read the Spring Scales
In air: 12 N. In water: 8 N. Find the buoyant force.
In air: 18 N. In water: 13 N. Find the buoyant force.
In air: 7 N. In water: 4.5 N. Find the buoyant force.
Part B – Force Diagrams
Gravity 6 N down, buoyant force 6 N up. Find the net force. Does it float, sink or rise?
Gravity 10 N down, buoyant force 4 N up. Find the net force. Does it float, sink or rise?
Gravity 3 N down, buoyant force 7 N up. Find the net force. Does it float, sink or rise?
Part C – Archimedes' Principle
A stone is lowered into water and pushes aside 250 mL of water. What is the buoyant force on it?
A block weighs 6 N in air. Underwater it displaces 400 mL of water. Find the buoyant force and what a spring scale reads underwater. Does it sink or float?
A toy boat weighs 3 N and floats. How much water does it displace?
A ball weighs 5 N and has a volume of 600 cm³. Will it float? Explain using Archimedes' principle.
Part D – True or False?
The buoyant force always pushes upward.
When an object floats, the buoyant force equals its weight.
Buoyant force on a rock depends on the rock's weight.
Salt water pushes up harder than fresh water.
An object sinks if buoyant force is more than its weight.
Objects weigh less in air than in water.
Part E – Buoyancy in Real Life
A cargo ship sits lower in the water when it's fully loaded. Explain why.
It's easier to float in the ocean than in a freshwater lake. Explain why.
A submarine can dive and surface. Explain how filling its tanks with water or air controls this.
Part F – Investigation
An object weighs 5.0 N in air. Lowered into water, the scale reads 5.0, 4.0, 3.0, 2.0, 1.0 and 1.0 N as it displaces 0, 100, 200, 300, 400 and 400 mL (the last one is deeper).
Complete the buoyant force column (5.0 N − scale reading).
Describe the pattern. How do the results support Archimedes' principle?
Predict the reading if the fully submerged object were in salt water instead. Explain.
Part G – Spot the Mistake
A student says, "A big rock feels a bigger buoyant force than a pebble, so the big rock will float." Explain the error.
Buoyancy Worksheet Answer Key
Open each part to check the buoyancy worksheet answers. The printable key is on the last two pages of the PDF.
Part A answers (Q1–3)
12 − 8 = 4 N
18 − 13 = 5 N
7 − 4.5 = 2.5 N
Part B answers (Q4–6)
Net force 0 N: floats
Net force 6 N down: sinks
Net force 4 N up: rises
Part C answers (Q7–10)
250 mL of water weighs 2.5 N, so the buoyant force = 2.5 N
Buoyant force = 4 N. Scale reads 6 − 4 = 2 N. It sinks (6 N > 4 N).
A floating object displaces its own weight of water: 3 N of water = 300 mL.
Fully under, it displaces 600 mL = 6 N > 5 N, so it floats (partly out of the water).
Part D answers (Q11–16)
True
True
False. It depends on the water displaced.
True
False. It sinks when its weight is greater.
False. They seem lighter in water.
Part E answers (Q17–19)
More weight means it must displace more water to float, so it sits deeper.
Salt water is denser. The same displaced volume weighs more, so it gives a bigger buoyant force.
Water in tanks: weight > buoyant force, so it sinks. Air in tanks: weight < buoyant force, so it rises.
Part F answers (Q20–22)
0, 1.0, 2.0, 3.0, 4.0, 4.0 N
Each 100 mL displaced adds 1 N: buoyant force = weight of water displaced. Deeper doesn't change it.
Less than 1.0 N. Salt water is denser, so the displaced water weighs more and the buoyant force is bigger.
Part G answer (Q23)
Its weight is bigger too. Rock is denser than water, so its weight always beats the buoyant force.
5 Common Buoyancy Mistakes
These come up often when marking this buoyancy worksheet. Part G targets the first one directly.
"A bigger buoyant force means it floats." It's the buoyant force compared with the weight that matters.
Thinking buoyant force depends on the object's weight. It depends on the fluid displaced.
Thinking deeper means more buoyant force. Once fully under, the displaced volume stops changing.
Saying objects "lose weight" in water. Gravity is the same; the water just pushes up.
Forgetting the direction. Buoyant force always points up; net force needs a direction too.
How to Use This Buoyancy Worksheet in Class
Lesson 1: Parts A–C of the buoyancy worksheet, alongside a spring-scale lab with a rock, a beaker and an overflow can.
Lesson 2: Parts D–G, after a build-a-foil-boat challenge to see how many pennies each boat can hold.
Extension: Make a Cartesian diver in a pop bottle and explain it with forces.
Yes. The worksheet and its answer key are free to download and print as one PDF, with no account needed.
What is Archimedes' principle?
The buoyant force on an object in a fluid equals the weight of the fluid the object pushes aside (displaces).
Why do objects weigh less in water?
The water pushes up on them with a buoyant force. A spring scale reads the weight in air minus the buoyant force, which is called the apparent weight.
How does a heavy steel ship float?
Its hull is shaped to push aside a huge volume of water. The ship sinks until the water it displaces weighs as much as the ship, and then the buoyant force balances its weight.
What grade is this buoyancy worksheet for?
It is written for Grade 8 science (the Fluids unit in Ontario) and also works as Grade 9 review.